It is known from EP 1 036 883 A to use standard sockets that are supplied with current from a three-phase generator. Electrical standard consumers can be connected to these standard sockets. In a road finisher, standard consumers require a standard net voltage and at least substantially a constant frequency. Here, electrical consumers refers to electrical tools, appliances and devices including by way of example, a hand drill, a pressure washer, a power screwdriver or also hand lamps or power supplies for mobile telephones, laptops etc. Such standard consumers are usually not required for operation of the road construction machines, but for use on the construction site in connection with the road laying operation or in cleaning or maintenance cycles and the like, for the road construction machine. Since the three-phase generator is more or less directly driven by the combustion engine of the road finishing machine, whose speed varies depending on the state of load, the frequency of the current supplied by the three-phase generator also varies. In the normal case, a frequency of approx. 50 Hz is reached at the three-phase generator and the sockets only at low engine speed, e.g. in the case of running at idle or when the idle speed is raised so that only in this tight engine speed range can conventional standard consumers be operated. If the engine speed is increased with respect to this speed range in which the standard consumers can be used, the output voltage of the three-phase generator is usually stabilized, however, the frequency rises in proportion to the speed. This increase in frequency would cause damage to the electrical standard consumers. Thus, it is known to activate the sockets on the road finisher only for the useable frequency for standard consumers, but to switch it off if the frequency becomes too low or too high. This means quite a considerable technical effort for the activation and deactivation of the standard sockets and bears the fundamental disadvantage of no longer being able to operate such standard consumers if the combustion engine does not run at a predetermined low speed. During normal working operation, the combustion engine runs at full load usually with a too high speed for electrical standard consumers so that they can then not be used. However, there is considerable need for permanently running certain electrical standard consumers at the road construction machine and not only in a low load state of the combustion engine. Although this requirement can be fulfilled in that no “conventional” electrical consumers are connected but only special consumers that can cope with high or varying frequencies, wherein special plug connectors are also used. However, this means a considerable additional expenditure. As an alternative, it is possible to use the usually provided 24-Volt on-board supply system of the road construction machine, i.e. supplied by a battery, for some electrical standard consumers. However, the 24-Volt on-board supply system is heavily loaded and rapidly drawn down by permanently or frequently switched-on standard consumers.
In a wheel loader known from JP 2001 10 36 05 A a converter must be connected to a supply line using electrical wheel motors, which can be operated independent of frequency and which are connected to a generator. A controlled transformer is connected downstream of the converter to which a socket is connected in which a predetermined frequency and a predetermined voltage is provided. Both the converter as well as the transformer are controlled separately by a central control device. The downstream connected transformer serves for generating a potential separation so that special consumers can be operated at the socket.